1. Field of the Invention
The present invention relates to dispensing guns for dispensing viscous materials which may be of a thick liquid or pasty nature.
2. Description of the Prior Art
Viscous material, such as mastic caulking material, is commonly supplied in a cartridge having a discharge nozzle. The cartridge can be mounted in a dispensing gun. An example of such a dispensing gun is described in British patent GB1555455. The gun has a plunger or push rod slidably mounted in a stock. The cartridge is mounted in a keep before the plunger which is advanced by means of a gripper plate and trigger assembly to force a piston, inside one end of the cartridge, forward to urge the material from the nozzle at the other end. The trigger and gripper plate are reset at the end of a dispensing stroke and the dispensing force can be reapplied. During dispensing the plunger is held against the end of the cartridge between trigger strokes by means of a locking plate.
By maintaining and reapplying a constant force on the trigger, it is found that it is possible to exert a high degree of control over the rate of discharge of the mastic material. However, while the gun is entirely acceptable as a dispensing tool for viscous materials in most situations, it is sometimes desirable to be able to halt the discharge immediately, when the hand releases the trigger. However, it is found that the body of the cartridge expands radially under a dispensing force. Similarly, if any pockets of gas are trapped in the cartridge they will compress during discharge of the material and expand once the dispensing force is removed. As the plunger is held by the locking plate substantially in the position at which the dispensing force was removed from the trigger, the contracting cartridge and any expanding pockets of gas tend to cause continued dispensing of the material when it is not required unless the locking plate is released manually.